1. Field of the Invention
The present invention is directed to a new composition, an enantiomer pair of an .alpha.-cyanobenzyl ester having useful pesticidal activity.
2. Description of the Prior Art
It is known from Pesticide Science, Vol 7, pages 273-77 (1976) and earlier work that the optical configuration of individual isomers of cyclopropanecarboxylate "pyrethroids" affects their insecticidal toxicity, and that in racemic mixtures of at least one pyrethroid ester derived from chrysanthemic acid there is mutual antagonism between pairs of isomers so that considerable masking of activity occurs.
Certain esters of substituted-phenylacetic acids are described in Belgian patent publication No. 801,946 as having pyrethroid-like properties, including pesticidal activity and low toxicity to mammals. It is further known how to prepare or to recover the S- optical form of such phenylacetic acids, e.g., Japanese patent publications Nos. 75/25,544 and 75/106,935, and that the pyrethroid esters of such S-acids were approximately twice as insecticidal as the corresponding racemate derived from both the S- and R- forms of such acids. The .alpha.-cyano-3-phenoxybenzyl ester diastereoisomer pair derived from the S-form of .alpha.-isopropyl-p-chlorophenylacetic acid can be separated into the single diastereoisomers and when tested for pesticidal activity, the stereoisomer (-)-S-.alpha.-cyano-3-phenoxybenzyl S-.alpha.-isopropyl-p-chlorophenylacetate was found to be up to several times more active than the double racemate derived from the R,S-alcohol and the R,S-acid and about twice as active as the diastereoisomer pair derived from the S-acid.
However, the procedures to prepare or recover the S- optical form of the phenylacetic acids is costly, time-consuming and requires considerable amounts of additional reagents, such as .alpha.-phenylethylamine, or highly caustic materials such as alkali or alkaline earth hydroxides or carbonates which are employed at temperatures in excess of 110.degree. C. One such process involves the resolution of a mixture of R,S-phenyl-acetic acid stereoisomers by stirring the stereoisomer mixture in warm aqueous ethanol with an equal number of moles of (-)-.alpha.-phenylethylamine to form a salt, cooling the mixture to form crystals of the salt of the S-acid, which is less soluble than the salt of the R-acid, separating the crystallized salt of the S-acid by filtration, washing the crystals with aqueous ethanol, and recovering the S-acid (a) using aqueous caustic and toluene to separate the S-acid, which dissolves in the aqueous phase, from the amine, which extracts into the toluene phase or (b) by addition of the salt to aqueous acid to precipitate the S-acid. The R-acid present in the original reaction mixture can be similarly recovered from the salt using aqueous ethanol and then aqueous caustic and toluene treatment or from the salt by treatment with acid. The R-acid is racemized and recycled for admixture with fresh feed mixture of R,S-phenylacetic acid stereoisomers.
By contrast, a highly active pesticidal stereoisomer mixture of a phenylacetate has been found which is much easier and economical to prepare than either the above diastereoisomer pair derived from the S--form of the acid or any single diastereoisomer, while providing improved pesticidal activity as compared to the racemate of the prior art.